The present invention relates to an electron beam writing or lithography system, and more particularly to an electron beam writing system using a cell projection method.
In a prior art variable shaped-beam writing system, as disclosed by Morita et al in "Proceedings of the Autumn Meeting of the Japan Society of Precision Engineering in Fiscal Showa 61 (1987)" pages 565-566, an electrostatic focus corrector is provided in a demagnification lens. Specifically, the entire beam optical path length is shortened to by half for the purpose of reducing the Coulomb effect, and a tip-planar type LaB.sub.6 is used as an electron gun, thereby providing sufficient brightness and emittance. The lens system is composed of a projection lens, a shaping lens, a demagnification lens and an objective lens. In order to realize high speed beam deflection scanning, all deflectors and correction lenses are of an electrostatic type. This is because the electrostatic-type instruments do not generate delays in beam positioning due to an eddy current, which is problematic for magnetic-type instruments. Both shaping deflection and sub-deflection are performed by two four-pole electrostatic deflectors. Main deflection is performed by an eight-pole deflector. Focus correction is performed by a cylindrical electrostatic lens (i.e., an electrostatic focus corrector) arranged in a demagnification lens.
The variable shaped-beam writing device forms the image of a first square aperture on a second square aperture to create any square beam (an electron beam with a sectional shape vertical to the axis of the beam being square). In such a beam writing system, a deflector focuses the square beam at several positions on a wafer. Then, focus correction is made to correct field curvature. The focus correction changes the magnification ratio so that the size of the square beam on a wafer will also change. This gives rise to an undesired result of deteriorating the accuracy of shot stitching on the wafer. Namely, the beam projection areas fluctuate on the writing object such as a wafer.
Moreover, high-speed focus correction is required to enhance the throughput. Thus, the voltage to be applied to a focus corrector is desired to be as small as possible. However the prior art does not take the above problems into consideration.